Low temperature plasma Si or SiGe for MEMS applications

ABSTRACT

A method is provided for making a MEMS structure ( 69 ). In accordance with the method, a CMOS substrate ( 51 ) is provided which has interconnect metal ( 53 ) deposited thereon. A MEMS structure is created on the substrate through the plasma assisted chemical vapor deposition (PACVD) of a material selected from the group consisting of silicon and silicon-germanium alloys. The low deposition temperatures attendant to the use of PACVD allow these materials to be used for MEMS fabrication at the back end of an integrated CMOS process.

FIELD OF THE INVENTION

[0001] The present invention relates generally to MEMS devices, and moreparticularly to low temperature methods for making MEMS devices out ofsilicon and silicon-germanium alloys.

BACKGROUND OF THE INVENTION

[0002] Advancements in micromachining and other microfabricationtechniques and processes have enabled the fabrication of a wide varietyof MicroElectroMechanical Systems (MEMS) and devices. These includemoving rotors, gears, switches, accelerometers, miniaturized sensors,actuator systems, and other such structures.

[0003] One important application of microfabrication is in thefabrication of RF MEMS switches. Such devices have several advantagesover their solid state counterparts. For example, RF MEMS switchesprovide lower insertion loss, higher isolation, better linearity, andlower power than solid state switches. RF MEMS devices are also usefulin a variety of applications. Thus, they can be used as tunablepreselectors and frequency synthesizers, and are also useful ascomponents in a variety of telecommunications devices and systems,including signal routing devices, impedance matching networks, andadjustable gain amplifiers.

[0004]FIG. 1 and FIG. 2 (the later of which is a side view of FIG. 1)depict a conventional RF MEMS switch 10. The switch includes acantilevered arm 20 which typically comprises an insulating material andwhich is attached to the substrate 12 by an anchor structure 14. Theanchor structure may be formed as a mesa on the substrate by depositionbuildup or through the selective removal or etching away of surroundingmaterial. A bottom electrode 16, which is typically grounded, and asignal line 18 are also formed on the substrate. The bottom electrodeand signal line typically comprise strips of a metal that is not easilyoxidized, such as gold. A gap exists between the signal line and thebottom electrode.

[0005] The actuating part of the switch comprises the cantilevered arm20 noted above. The cantilevered arm forms a suspended microbeam whichis attached at one end to the top of the anchor structure and whichextends over and above the bottom electrode and the signal line disposedon the substrate. An electrical contact 22, which also typicallycomprises a metal such as gold that does not oxidize easily, is formedon the end of the cantilever arm that is removed from the anchorstructure. The electrical contact is positioned on the bottom side ofthe cantilever arm so as to face the top of the substrate over and abovethe signal line.

[0006] A top electrode 24, typically comprising a metal such as aluminumor gold, is formed atop the cantilever arm. The top electrode startsabove the anchor structure and extends along the top of the cantileveredarm to end at a position above the bottom electrode. The cantileveredarm and top electrode are broadened above the bottom electrode (which isitself broadened) to form a capacitor structure 26. The capacitorstructure is provided with a grid of holes to reduce its mass.

[0007] In operation, the switch is normally in an “Off” position asshown in FIG. 2. With the switch in the off-state, the signal line is anopen circuit due to the gap between the electrical contact and thesignal line. The switch is actuated to the “On” position by applicationof a voltage on the top electrode. With a voltage on the top electrode,electrostatic forces attract the capacitor structure (and cantileverarm) toward the bottom electrode. Actuation of the cantilevered armtoward the bottom electrode, as indicated by arrow 11, causes theelectrical contact to move against the signal line, thereby closing thegap and placing the signal line into the on-state (i.e., closing thecircuit).

[0008] One problem encountered in devices of the type depicted in FIGS.1 and 2 relates to the mismatch in coefficients of thermal expansion(CTEs) between the materials used for certain components of the device.In particular, in the case of RF MEMS switches, a thermal mismatchtypically exists between the top electrode (which, as noted above, istypically made out of a metal such as Au) and the cantilevered arm(which is usually made out of a material such as silicon oxynitride(SiON)). As a result, the movable portion of the switch tends to becomepermanently distorted during the thermal cycles that occur after releaseand during the packaging process, thus leading to changes in theoperating characteristics of the switch and, in many cases, switchfailure.

[0009] A variety of other materials have been used in MEMS fabricationprocesses, some of which have CTEs that more closely match the CTE ofSiON. However, the use of many of these materials in the top electrodeof an RF MEMS switch has been precluded by the processing considerationsattendant to conventional fabrication methodologies. Thus, for example,silicon and silicon/germanium alloys have been used as structuralelements in MEMS processes using LPCVD, and have a number of desirableproperties. However, the maximum processing temperature for a typical RFMEMS switch is limited to about 350° C. (due primarily to the presenceof the sacrificial layer, which is typically made out of a polyimide ora similar thermally sensitive material), which is well below thedeposition temperatures of about 550° C. that are required for siliconor silicon-germanium alloys in an LPCVD or epitaxial process.

[0010] Processing temperature considerations have likewise precluded theuse of materials such as silicon and silicon/germanium alloys in otherMEMS applications, in spite of the desirable physical and electricalproperties that these materials have. Such applications include, forexample, the fabrication of MEMS devices integrated with CMOS(Complimentary Metal Oxide Semiconductor) structures such as sensors andactuators. CMOS structures are very effective device configurations forthe implementation of digital functions, due to their low powerconsumption and dissipation and the minimization of their current in theoff-state. With commercial CMOS-compatible micromachining,microstructures and support circuitry can coexist on the same substrate,and thus can be fabricated in an integrated process.

[0011] However, in order to ensure proper integration into a CMOSprocess and good portability between generations of CMOS, it ispreferable to integrate MEMS fabrication into the backend of a CMOSprocess. This requires formation of the MEMS structures after theinterconnect metal has already been deposited. However, the presence ofthe interconnect metal on the substrate requires that the substrate notbe exposed to temperatures in excess of 450° C.; these temperatures areagain well below the deposition temperatures of about 550° C. that arerequired for silicon or silicon-germanium alloys in an LPCVD orepitaxial process. Hence, the use of these materials in backendprocessing of a CMOS device is precluded. Although it may be possible insome process flows to circumvent this problem by integrating the MEMSfabrication into the beginning or middle of a CMOS process, this isundesirable in that it limits the portability of the process betweengenerations of CMOS.

[0012] There is thus a need in the art for a low temperature method formaking MEMS devices or components thereof out of silicon orsilicon/germanium alloys. There is also a need in the art for a methodof fabricating MEMS structures or components based on these materialswhich can be integrated into the backend of a CMOS process, and whichcan be used to fabricate sensors and actuators. There is further a needin the art for an RF MEMS device, and a method for making the same, inwhich the CTE of the top electrode and cantilevered arm are closelymatched. These and other needs are met by the devices and methodologiesdisclosed herein.

SUMMARY OF THE INVENTION

[0013] In one aspect, a method for making a MEMS device is providedherein. In accordance with the method, a substrate is provided, and aMEMS structure or component thereof is created on the substrate throughthe Plasma Assisted Chemical Vapor Deposition (PACVD) of a materialselected from the group consisting of silicon and silicon-germaniumalloys. The low temperatures attendant to PACVD allow these materials tobe used in fabrication processes where their use would have previouslybeen precluded by processing considerations, and also allows thesematerials to be doped in situ. In accordance with this method, PACVD maybe used to fabricate MEMS structures (or components thereof) on avariety of substrates and in a variety of applications. Thus, forexample, PACVD may be used to fabricate MEMS structures on CMOSsubstrates (in which case the MEMS structure could be, for example, asensor or actuator). PACVD may also be used to fabricate MEMS structuresor components (including, for example, electrode and structuralelements) in an RF MEMS fabrication process.

[0014] In another aspect, a method for making a MEMS structure isprovided. In accordance with the method, a CMOS substrate is providedhaving interconnect metal deposited thereon. The interconnect metal maycomprise, for example, gold or aluminum. A MEMS structure or componentthereof is created on the substrate through the plasma assisted chemicalvapor deposition of a material selected from the group consisting ofsilicon and silicon-germanium alloys. The plasma assisted chemical vapordeposition typically occurs at a temperature of less than about 450° C.,preferably less than about 400° C., more preferably less than about 350°C., even more preferably less than about 300° C., and most preferablyless than about 250° C. If desired, the material may be doped as it isdeposited.

[0015] In still another aspect, a method for manufacturingmicroelectromechanical sensors and actuators is provided herein. Inaccordance with the method, a CMOS substrate is provided having at leasta first surface region thereon comprising a first material selected fromthe group consisting of silicon, glass and gallium arsenide, and atleast a second surface region thereon comprising a second materialselected from the group consisting of silicon oxide and polyimide. Alayer of a third material is formed over the substrate which extendsover at least a portion of the first and second regions, wherein thethird material is selected from the group consisting of silicon andsilicon-germanium alloys, and wherein the layer of the third material isformed at a temperature of less than about 450° C., more preferably lessthan about 350° C., even more preferably less than about 300° C., andmost preferably less than about 250° C. through a plasma assistedchemical vapor deposition process. At least a portion of the secondmaterial is removed from underneath the layer of the third material soas to form a micromechanical deflection element comprising the thirdmaterial.

[0016] In yet another aspect, a method is provided herein for making anRF MEMS switch. In accordance with the method, a substrate is providedhaving a signal line or other circuitry defined thereon for supportingan RF MEMS switch. A sacrificial layer is applied to at least a portionof the substrate. A structural element of an RF MEMS switch is thenformed over the sacrificial layer, and a top electrode is formed on thestructural element through the use of a plasma assisted chemical vapordeposition process. The top electrode is formed at a temperature that istypically less than about 400° C., more preferably less than about 350°C., even more preferably less than about 300° C., and most preferablyless than about 250° C. The top electrode comprises a material selectedfrom the group consisting of silicon and silicon-germanium alloys. Atleast a portion of the sacrificial layer is then removed from underneaththe structural element so as to release the element.

[0017] In another aspect, an RF MEMS switch is provided which has anelectrode comprising a material selected from the group consisting ofsilicon and silicon/germanium alloys. The switch preferably comprises acantilevered arm, and the electrode is preferably disposed on the top ofthe cantilevered arm. A second electrode is preferably disposed belowthe cantilevered arm.

[0018] In still another aspect, devices are disclosed which may be madeusing the above noted methodologies.

[0019] These and other aspects are described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a top view of a prior art RF MEMS;

[0021]FIG. 2 is a side view of a prior art RF MEMS;

[0022]FIG. 3 is an illustration of a PACVD reactor suitable for use inthe practice of the methodologies described herein;

[0023] FIGS. 4-15 are cross-sectional illustrations of one possiblemethod for making an RF MEMS in accordance with the teachings herein;

[0024]FIG. 16 is a graph of deposition rate as a function of temperaturefor PACVD of silicon at two different power settings;

[0025]FIG. 17 is a graph of deposition rate as a function of pressurefor PACVD of silicon; and

[0026]FIG. 18 is a graph of beam deflection as a function of appliedvoltage for MEMS having both Au and silicon electrodes.

DETAILED DESCRIPTION

[0027] Chemical Vapor Deposition (CVD) has been widely used in the artto generate films, coatings, and structures from gaseous precursors.Several different species of CVD are currently known, including, forexample, Low Pressure Chemical Vapor Deposition (LPCVD).

[0028] One type of CVD that has been developed fairly recently is PlasmaAssisted Chemical Vapor Deposition (PACVD), which is also referred to asPlasma Enhanced Chemical Vapor Deposition (PECVD). In PACVD, the energyfor inducing the reaction in the precursor that generates the coatingmaterial comes from the collision of plasma ions and electrons with theprecursor. The plasma itself is typically created through the use ofmicrowaves or electric fields. Since PACVD does not rely on thermallyinduced reactions in the precursor, the temperature experienced by asubstrate in a PACVD process is often lower than the depositiontemperatures experienced by a substrate in other commonly useddeposition processes such as LPCVD or epitaxy.

[0029] It has now been found that PACVD can be successfully used in aMEMS fabrication process for silicon or silicon/germanium alloys tosubstantially lower the temperature experienced by the substrate in sucha process. In particular, PACVD may be used to deposit silicon andsilicon/germanium alloys at temperatures in the range of 200-300° C., ascompared to the deposition temperatures of about 550° C. that aretypically required for these materials in a MEMS fabrication processbased on LPCVD or epitaxy. This has allowed MEMS devices, and componentsthereof (including anchor portions, microbeams, cantilevered arms andother MEMS structures and components), which are based on silicon andsilicon/germanium alloys to be formed on a variety of substrates thatare incompatible with conventional fabrication methodologies (such asLPCVD or epitaxial processes) due to temperature restrictions. Thus, forexample, PACVD may be used to form electrodes (including top electrodes)on RF MEMS switches. PACVD may also be used to form accelerometers,pressure sensors, and components of these devices on CMOS substrates,where the low temperatures it affords permits its use in the backend ofan integrated process.

[0030] A PACVD reactor suitable for use in the fabrication of MEMSstructures and components in accordance with the teachings herein isdepicted in FIG. 3. As shown therein, the reactor 31 consists of areaction chamber 33 within which a substrate 35 may be mounted. Thereaction chamber is maintained in a near vacuum state by a pump 37. Thesubstrate may be maintained at a suitable deposition temperature withinthe reaction chamber through the use of a substrate heater 39. Thetemperature of the substrate is monitored by means of a thermocouple 41.In the applications described herein, the substrate will typically bemaintained at a temperature of less than about 450° C., more preferablyless than about 350° C., even more preferably less than about 300° C.,and most preferably less than about 250° C.

[0031] The PACVD reactor relies on the decomposition of precursor gasesto achieve deposition of materials onto a substrate. For example, silane(SiH₄) may be used as a precursor gas in the deposition of silicon. Theprecursor gases are supplied to a gas reservoir 43 which is in opencommunication with the reaction chamber. The flow of the precursor gasesinto the gas reservoir is regulated by a gas manifold 45. As theprecursor gases flow into the reaction chamber, they react with a plasmagenerated by an RF electrode 47 and undergo decomposition reactions toyield silicon and other solid reaction byproducts that are depositedonto the surface of the substrate as a film.

[0032] In addition to the low deposition temperatures it affords,another significant advantage PACVD offers over other depositionmethodologies in the context of MEMS fabrication is that doping can beconducted in situ, that is, the material may be doped as it isdeposited. This may be accomplished by incorporating a suitable gas intothe gas feed stream of the PACVD that can undergo a decompositionreaction in the deposition chamber to yield a suitable dopant. Thus, forexample, phosphine (PH₃) may be introduced into the feed stream wheren-doping is desired, and diborane (B₂H₆) may be introduced wherep-doping is desired. These gases undergo decomposition to yieldelementary phosphorous and boron, respectively, which are thenincorporated into the layer being formed. By contrast, conventionalapproaches often require doping of a layer or structure after the layeror structure has been formed. Such post-deposition doping can damage thesubstrate, and may also result in undesirable doping gradients.

[0033] Once the dopant is incorporated into the structural material, itmay be activated by various means. Some degree of doping activation willoccur during the PACVD process itself. However, total activation may beachieved by various other means. Laser annealing is a particularlydesirable method for total dopant activation, since it can be achievedwithout exposing the substrate to high temperatures.

[0034] FIGS. 4-15 illustrate one possible fabrication sequence by whichan RF MEMS in accordance with the teachings herein can be made. Thoughthe structure made by this process differs in design from the prior artstructure shown in FIGS. 1 and 2, it will be appreciated that the samegeneral methodology could also be used to produce a structure similar tothat depicted in FIGS. 1 and 2 but in which the top metal electrode isreplaced with a silicon electrode.

[0035] As shown in FIG. 4, a substrate 51 is provided which may comprisea material such as high resistivity silicon, silicon oxide, or GaAs. Alayer 53 of SiO₂ is disposed on the substrate by, e.g., thermaloxidation, typically to a thickness of about 0.8 microns. As shown inFIG. 5, metallization 55 for supporting the RF MEMS switch (e.g., forsignal input/output and/or for grounding purposes) is then defined onthe silicon oxide layer by forming a first photo mask (not shown) overthe silicon oxide layer, followed by evaporation and liftoff. Apolyimide spacer 57 is then formed as shown in FIG. 6.

[0036] Next, a first SiON etch mask 59 is formed over the polyimidelayer by PACVD, followed by formation of a second photo mask (not shown)for defining recesses in the structure. A contact recess 61 and acontact post recess 63 are then partially etched through the sacrificialmaterial into the structure, followed by removal of the recess mask toyield the structure shown in FIG. 7.

[0037] As shown in FIG. 8, a second SiON etch mask 65 is formed over thefirst SiON etch mask by PACVD. A third photo mask (not shown) is formedover the structure, and suitable etching techniques are used to definean anchor via recess 67 in the structure by etching completely throughlayer 57. The first and second SiON masks are then removed from thestructure as shown in FIG. 9.

[0038] As shown in FIG. 10, a travel stop 69 and shorting bar 71 areformed in the contact and contact post recesses, respectively, whichwere previously defined in the structure through formation of a fourthphoto mask (not shown), followed by evaporation and lift-off. As shownin FIG. 11, the portion or layer giving rise to the cantilever portion73 of the device is then formed out of low stress SiON by PACVD. Asshown in FIG. 12, the patterned top electrode 75, which may comprise,for example, silicon or a silicon/germanium alloy, is then formed on thecantilevered arm by formation of a fifth photo mask (not shown),followed by evaporation and lift-off. The use of PACVD to form the topelectrode is advantageous here not only because of the low temperaturesit affords, but because the silicon or silicon/germanium alloy may beadvantageously doped in-situ.

[0039] As shown in FIG. 13, a number of etch holes 77 are formed in theSiON layer through the use of a sixth photo mask (not shown), or aselective etch, followed by etching. An aperture 79 is defined in thebottom of the anchor via recess in a similar manner to expose theunderlying metallization.

[0040] As shown in FIG. 14, an anchor pad 81 is formed in the anchor viarecess by formation of a seventh photo mask (not shown), followed byevaporation and lift-off. The resulting anchor pad forms an electricalconnection between the top electrode 75 and the bond pads 55. The finalstructure 83 is then released by chemical removal of the polyimide layer57, as shown in FIG. 15.

[0041] The parameters of the PACVD process used to create silicon orsilicon-germanium films in accordance with the teachings herein may bemanipulated to modify the crystallinity of the resulting film. Forexample, pulsed gas technologies may be employed to producenano-crystalline PACVD silicon films for use in RF MEMS and variousother MEMS devices. Such nano-crystalline films may be particularlyadvantageous in some processes for in situ doping to produce filmshaving a lower resistivity.

[0042] The devices and methodologies described herein may be furtherunderstood with reference to the following non-limiting examples.

EXAMPLES 1-42

[0043] These examples illustrate the use of PACVD to achieve the lowtemperature deposition of silicon on a substrate.

[0044] A series of silicon wafer substrates were placed in the reactionchamber of a Novellus Concept One Plasma Enhanced Chemical VaporDeposition System (available commercially from Novellus Systems, Inc.,San Jose, Calif.). The system was equipped with a vacuum pump and gasinlets adapted to provide a steady flow of silane as the precursor andhelium as a carrier gas. The system was also equipped with a temperaturecontrol mechanism to maintain a steady temperature in the reactionchamber, and a microwave source having an adjustable power output. Theflow rates of the precursor and carrier gases, the wattage of themicrowave source, and the temperature and pressure of the reactionchamber for each experiment are indicated in TABLE 1. TABLE 1 PLASMASILICON DEPOSITION DATA He Flow SiH4 Flow HFRF Temp. Deposition PressureThickness Dep. Rate Stress Si—H Bonds EXAMPLE (SLM) (SLM) (Watts) (° C.)Time (Torr) (Å) (ang/min) (e⁹ Dynes/cm²) (e²¹ At/cc) 1 2 0.2 200 250 5002 2456 295 −5.12 No Data 2 2 0.5 400 250 180 2 2079 693 −0.86 No Data 34 0.5 200 250 500 2 1895 227 −6.64 No Data 4 2 0.5 200 250 500 2 1878225 0.16 No Data 5 2 0.2 400 250 700 2 2111 181 −0.60 No Data 6 4 0.2200 250 500 2 1951 234 −9.76 No Data 7 2 0.2 200 300 500 2 2391 287−5.98 8.08 8 2 0.5 400 300 180 2 2252 751 −3.50 9.84 9 4 0.5 200 300 5002 2757 331 −5.31 6.86 10 2 0.5 200 300 500 2 2283 274 −2.56 7.86 11 20.2 400 300 700 2 1965 168 −7.39 11.60 12 4 0.2 200 300 500 2 3522 423−5.84 6.64 13 2 0.2 200 350 500 2 1674 201 −9.83 9.09 14 2 0.5 400 350180 2 1961 654 −4.45 7.62 15 4 0.5 200 350 500 2 2785 334 −5.87 6.15 162 0.5 200 350 500 2 2121 255 −4.11 7.05 17 2 0.2 400 350 700 2 1960 168−8.09 9.30 18 4 0.2 200 350 500 2 2004 240 −10.30 9.57 19 2 0.2 200 350654 1.5 2141 197 −10.60 7.75 20 2 0.5 400 350 300 1.5 2109 422 −9.569.61 21 4 0.5 200 350 538 1.5 2077 232 −9.66 8.97 22 2 0.5 200 350 3701.5 2020 328 −7.74 7.34 23 2 0.2 400 350 483 1.5 2144 266 No Data 9.6424 4 0.2 200 350 747 1.5 2014 162 −12.00 10.1 25 2 0.2 200 350 474 2.51975 250 −8.11 7.57 26 2 0.5 400 350 114 2.5 1884 992 No Data 8.59 27 40.5 200 350 345 2.5 2027 353 −6.94 7.38 28 2 0.5 200 350 179 2.5 1821610 −3.29 8.36 29 2 0.2 400 350 215 2.5 1672 467 No Data 11.60 30 4 0.2200 350 589 2.5 1925 196 −1.03 8.90 31 2 0.2 200 350 300 2 1038 208−9.65 No Data 32 2 0.2 200 350 300 2 1039 208 −10.20 No Data 33 2 0.5400 350 77 2 706 554 −5.85 No Data 34 2 0.5 400 350 77 2 702 551 −6.60No Data 35 4 0.5 200 350 180 2 756 252 −7.85 No Data 36 4 0.5 200 350180 2 757 252 −8.09 No Data 37 2 0.5 200 350 235 2 2245 573 −4.19 NoData 38 2 0.5 200 350 235 2 2262 578 −3.69 No Data 39 2 0.2 400 350 3582 1864 313 −11.4 No Data 40 2 0.2 400 350 358 2 1886 317 No Data No Data41 4 0.2 200 350 240 2 698 175 −10.90 No Data 42 4 0.2 200 350 240 2 699175 −11.00 No Data

[0045] As seen from the data depicted in TABLE 1, it was possible tovary the parameters of the deposition process, within a temperaturerange that did not exceed 350° C. (the maximum temperature range for RFMEMS switches), to achieve deposition rates that varied from about 160angstroms/min to about 1000 angstroms/min. The layer thickness of thesilicon deposited varied from about 700 angstroms to greater than about3500 angstroms, and the layers formed had low residual stress. Thesedeposition rates and layer thicknesses are sufficient for a commercialMEMS fabrication process.

[0046] Some trends are also discernible in the data of TABLE 1 in termsof the effect of temperature, pressure and RF power on deposition rate.To better illustrate these trends, selected data points from TABLE 1 aredepicted graphically in FIGS. 16-17. Thus, FIG. 16 shows the depositionrate as a function of temperature (at a pressure of 2 Torr), andincludes data points at an RF power of 200 watts and 400 watts. Asindicated by FIG. 16, RF power was seen to have a significant effect ondeposition rates, with a higher power resulting in a higher depositionrate at a given temperature. Temperature was seen to have a noticeable,though somewhat lesser, effect on deposition rate for a given RF power.

[0047]FIG. 17 is a graph of deposition rate as a function of thepressure in the reaction chamber at a temperature of 350° C. As seen inthis graph, deposition rates varied linearly with pressure over theranges tested.

[0048] Referring again to the data of TABLE 1, the effective volume ofSi—H bonds in the deposited layer suggests that the use of PACVD forsilicon deposition using silane as a precursor leads to the formation ofa layer of amorphous hydrogenated silicon. Amorphous hydrogenatedsilicon has material properties that are significantly better than thoseof pure amorphous silicon, due to the high defect density of the latercompared to the former and the effect that those defects have inallowing trapped charge carriers to easily recombine. Hence, the use ofPACVD in MEMS fabrication is seen to have benefits beyond thoseresulting from the low temperatures that the process affords.

[0049] These results indicate that PACVD can be used to deposit siliconat commercially feasible rates and layer thicknesses in a MEMSfabrication process. Moreover, the temperatures that the substrate isexposed to in this process are sufficiently low to allow its use in thefabrication of RF MEMS switches, or the fabrication of MEMS structuresat any point in a conventional CMOS process, including the back end.Also, the physical properties of the silicon deposited by the PACVDprocess are superior, in terms of their electrical properties, tosilicon layers produced by some other methods.

COMPARATIVE EXAMPLE 1

[0050] This example illustrates the thermal properties of a switch madewith a solid gold top electrode.

[0051] A switch of the type depicted in FIG. 15, and made in accordancewith the general methodology depicted in FIGS. 4-15, was fabricated,except that the top electrode was solid gold and had a thickness of 0.2microns. The modulus and coefficient of thermal expansion of the Auelectrode were determined (at a temperature within the range of 60-80°C.) and are set forth in TABLE 2.

EXAMPLES 43-45

[0052] The following examples illustrate the thermal stability of an RFMEMS made by way of PACVD.

[0053] In EXAMPLE 43, a switch was made as in COMPARATIVE EXAMPLE 1,except that the gold electrode was replaced with an undoped siliconelectrode having a thickness of 0.3 microns. The modulus and coefficientof thermal expansion of the electrode were again determined and are setforth in TABLE 2.

[0054] In EXAMPLE 44, a switch was made as in COMPARATIVE EXAMPLE 1,except that the gold electrode was replaced with a silicon electrodehaving a thickness of 0.3 microns that was doped with boron. The modulusand coefficient of thermal expansion of the electrode were againdetermined and are set forth in TABLE 2.

[0055] In EXAMPLE 45, a switch was made as in COMPARATIVE EXAMPLE 1,except that the gold electrode was replaced with a silicon electrodehaving a thickness of 0.3 microns that was doped with phosphorous. Themodulus and coefficient of thermal expansion of the electrode were againdetermined and are set forth in TABLE 2. TABLE 2 Physical Properties ofTop Electrode of Switches Material of Upper Modulus CTE ThicknessExample Electrode (GPa) (ppm/° C.) (microns) COMPARATIVE Au 80 15 0.2EXAMPLE 1 EXAMPLE 43 Undoped 189-238 0.87 0.3 Silicon EXAMPLE 44 B-doped115-175 0.87 0.3 Silicon EXAMPLE 45 P-doped 194-220 0.87 0.3 Silicon

[0056] As the results of TABLE 2 indicate, the upper electrodes on theswitches of EXAMPLES 43-45 had coefficients of thermal expansion thatwere much lower than the coefficient of thermal expansion of the Auelectrode, thus demonstrating the improved thermal stability of theswitches of EXAMPLES 43-45 compared to the switch of COMPARATIVEEXAMPLE 1. The upper electrodes on the switches of EXAMPLES 43-45 alsohad somewhat higher moduli, and therefore somewhat lower sensitivities,compared to the electrode of the switch of COMPARATIVE EXAMPLE 1.However, both of these parameters (modulus and sensitivity) were withinacceptable ranges for most applications, nor was any attempt made tooptimize these results.

[0057]FIG. 18 illustrates the effect on the actuation voltage whensilicon is substituted for gold in a switch of the type depicted in FIG.12. The beam deflection as a function of applied voltage was measuredfor the switches of COMPARATIVE EXAMPLE 1 and EXAMPLE 43. As showntherein, the actuation voltage (that is the voltage required for thesignal line and the adjacent electrical contact to come into electricalcontact with each other) for the switch of COMPARATIVE EXAMPLE 1 wasabout 46-48 V, compared with 50-55 V for the switch of EXAMPLE 43, thusdemonstrating that a switch having an acceptable actuation voltage canbe made based on silicon. The slight increase in actuation voltage isattributable at least in part to the higher modulus of silicon and thegreater thickness of the silicon electrode of EXAMPLE 43 compared to thegold-based electrode of COMPARATIVE EXAMPLE 1.

[0058] A low temperature method for making MEMS devices out of siliconor silicon/germanium alloys has been provided herein, along with amethod of fabricating MEMS structures based on these materials which canbe integrated into the backend of a CMOS process, and which can be usedto fabricate sensors and actuators. An RF MEMS device, and a method formaking the same, have also been provided in which the CTE of the topelectrode and cantilevered arm are closely matched such that the deviceexhibits substantially improved thermal stability compared to prior artdevices.

[0059] The above description of the present invention is illustrative,and is not intended to be limiting. It will thus be appreciated thatvarious additions, substitutions and modifications may be made to theabove described embodiments without departing from the scope of thepresent invention. Accordingly, the scope of the present inventionshould be construed in reference to the appended claims.

What is claimed is:
 1. A method for making a MEMS structure, comprisingthe steps of: providing a CMOS substrate having interconnect metaldeposited thereon; and creating a MEMS structure on the substratethrough the plasma assisted chemical vapor deposition of a materialselected from the group consisting of silicon and silicon-germaniumalloys.
 2. The method of claim 1, wherein the MEMS structure is asensor.
 3. The method of claim 1, wherein the MEMS structure is anactuator.
 4. The method of claim 1, wherein the interconnect metalcomprises gold.
 5. The method of claim 1, wherein the material is dopedas it is deposited.
 6. The method of claim 1, wherein the material isdeposited at a temperature of less than about 450° C.
 7. The method ofclaim 1, wherein the material is deposited at a temperature of less thanabout 350° C.
 8. The method of claim 1, wherein the material isdeposited at a temperature of less than about 300° C.
 9. The method ofclaim 1, wherein the material is deposited at a temperature of less thanabout 250° C.
 10. The method of claim 1, wherein the material comprisesamorphous hydrogenated silicon.
 11. A method for manufacturing amicroelectromechanical deflection element, comprising the steps of:providing a CMOS substrate having at least a first surface regionthereon comprising a first material selected from the group consistingof silicon, silicon oxide and gallium arsenide, and at least a secondsurface region thereon comprising a second material selected from thegroup consisting of silicon oxide and polyimide; forming a layer of athird material which extends over at least a portion of the first andsecond regions, wherein the third material is selected from the groupconsisting of silicon and silicon-germanium alloys, and wherein thelayer of the third material is formed at a temperature of less thanabout 450° C. through a plasma assisted chemical vapor depositionprocess; and removing at least a portion of the second material fromunderneath the layer of the third material so as to form amicromechanical deflection element comprising the third material. 12.The method of claim 11, wherein the layer of the third material isformed at a temperature of less than about 400° C.
 13. The method ofclaim 11, wherein the layer of the third material is formed at atemperature of less than about 350° C.
 14. The method of claim 11,wherein the layer of the third material is formed at a temperature ofless than about 300° C.
 15. The method of claim 11, wherein the layer ofthe third material is formed at a temperature of less than about 250° C.16. The method of claim 11, wherein the third material comprisessilicon.
 17. The method of claim 11, wherein the third materialcomprises germanium.
 18. The method of claim 11, wherein the third layeris doped as it is formed.
 19. The method of claim 18, wherein the dopantused to dope the third layer is boron or phosphorous.
 20. The method ofclaim 11, wherein the micromechanical deflection element is a componentof a sensor.
 21. The method of claim 11, wherein the micromechanicaldeflection element is a component of an actuator.
 22. The method ofclaim 11, wherein the third material comprises amorphous hydrogenatedsilicon.
 23. A method for making an RF MEMS switch, comprising the stepsof: providing a substrate, the substrate having circuitry definedthereon for supporting an RF MEMS switch; applying a sacrificial layerto at least a portion of the substrate; forming a structural element ofan RF MEMS switch that extends over at least a portion of thesacrificial layer; forming, through the use of a plasma assistedchemical vapor deposition process, an electrode on a surface of thestructural element, the electrode comprising a material selected fromthe group consisting of silicon and silicon-germanium alloys; andremoving at least a portion of the sacrificial layer from underneath thestructural element so as to release the structural element.
 24. Themethod of claim 23, wherein the structural element is formed bydepositing and patterning a layer of SiON over the sacrificial layer.25. The method of claim 23, wherein the sacrificial layer comprises apolyimide.
 26. The method of claim 23, wherein the circuitry comprisesgold.
 27. The method of claim 23, wherein the structural element isformed at a temperature of less than about 350° C.
 28. The method ofclaim 23, wherein the structural element is formed at a temperature ofless than about 300° C.
 29. The method of claim 23, wherein thestructural element is formed at a temperature of less than about 250° C.30. The method of claim 23, wherein the material comprises amorphoushydrogenated silicon.